Method of transmitting speech in ciphered form and arrangement for carrying out the method



March 3l, 1970 A. C- JACQBAEUS METHOD OF TRANSMITTING SPEECH IN CIPHERED FORM AND ARRANGEMENT FOR CARRYING OUT THE METHOD Filed May 29, 1968 Pc'M PCM c coder IP fa decoder M Dec! hen PCM v decoder P 29 A cader Q:

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INVENTOR.

\N'rc N Qua 51' n: smoghtus BY on. MA J A wr'roknevs United States Patent 3,504,286 METHOD OF TRANSMITTING SPEECH IN CIPHERED FORM AND ARRANGEMENT FOR CARRYING OUT THE METHOD Anton Christian .l'acobaeus, Stockholm, Sweden, assignor to Telefonaktiebolaget L M Ericsson, Stockholm, Sweden, a corporation of Sweden Filed May 29, 1968, Ser. No. 732,994 Claims priority, application Sweden, June 21, 1967,

8,919/ 67 Int. Cl. H04k 1/04 US. Cl. 32532 11 Claims ABSTRACT OF THE DISCLOSURE The invention concerns a method of transmitting speech in ciphered form from a transmitter to a receiver and an arrangement for carrying out this method.

Known methods of transmitting ciphered speech are based on a digitalization of the speech signal. The digital information is treated with common ciphering methods and the speech transmission takes place in digital form. The digitalization can be carried out either by means of a vocoder, in which case the terminal equipments will be fairly expensive, or by pulse code modulation or delta modulation.

An object of the invention is to provide a new method of transmitting ciphered speech, according to limited demands on band width and simultaneously with inconsiderable costs for the terminal equipments.

The method according to the inventon is substantially characterized in that the speech signal is converted into a pulse code which is enciphered. The enciphered pulse code is converted into a speech signal and the speech signal, together with a synchronizing signal, is transmitted to a receiver where it is converted into a pulse code. The code is deciphered and the restored pulse code is converted into a speech signal.

An arrangement for carrying out the method comprises: a sender which includes a coder which converts the speech signal into a pulse code, a ciphering arrangement which transforms the obtained pulse code values representing the sampling values into ciphered pulse code values, a modulating device for adding a synchronizing signal to the ciphered signal, and a decoder which converts the ciphered digital signals into a speech signal; and a receiver which includes a coder for converting the obtained ciphered speech signals into a pulse code, a deciphering device which in response to the digital values represented by the pulse code restores the pulse code corresponding to the original speech, and a decoder which restores the pulse code to the original speech signal.

The invention will be further described by means of an embodiment with reference to the accompanying drawing in which FIG. 1 shows a system consisting of a transmitter and a receiver, for carrying out the method according to the invention by means of the PCM (pulse code modulation) principle and FIG. 2 is a diagram showing the synchronizing signal used in the system of FIG. 1.

The speech signal is supplied to a PCM-coder 3 through a suppressed frequency band filter 1 having a suppressed frequency band around the frequency of 2 kHz. (the purpose of which will be explained below) and a low-pass filter 2 with an upper limit frequency of 4 kHz. A sampling of the speech signal is carried out with a frequency of 8 kHz. Consequently, speech signals between 04 kHz. can be converted into PCM-signals in the PCM-coder 3 in a known manner. The PCM-signals are then supplied to a ciphering device 4 known, per se, in which they are converted into ciphered form. These signals are then supplied through a modulator 5, the purpose of which will be explained below, to a PCM-decoder 6 where they are converted into a voice frequency signal. Finally the signal will pass, before being transmitted, through a low-pass filter 7 with the upper limit frequency of 4 kHz. At the input of the PCM-coder an incoming speech signal b1 is shown diagrammatically and at the output of the PCM- decoder the ciphered form b2 of the speech signal is indicated.

A On the receiver side the incoming signal is supplied through a low-pass filter 8 with the upper limit frequency of 4 kHz. and a suppressed frequency band filter 9 to ,a PCM-coder 10. The PCM-signals are restored to their original form by means of a deciphering device 11 and the deciphered PCM-signals are supplied to a PCM-decoder 12 which restores the original voice frequency signal. The restored signals are supplied to a utilization device, for example a telephone receiver (not shown) through a low-pass filter 13.

In order to allow the arrangement to function, the transmission medium must of course be linear and without transmit time distortion. Furthermore, synchroniza tion is necessary between the sampling on the transmitter side and on the receiver side. The synchronization is carried out according to the invention in such a way that there is inserted into the voice frequency band a frequency corresponding to a submultiple of the sampling frequency and the zero crossings of this frequency are used for synchronization. This can be carried out by sacrificing a small part of the available voice frequency band. According to the example, the synchronizing frequency is chosen to 2.kHz. at a sampling frequency of 8 kHz. The purpose of the filter 1 is to remove a narrow band located around the frequency of 2 kHz., from the frequency spectrum of the original speech signal, in order to allow a synchronizing frequency of 2 kHz. to be mixed into the ciphered pulse code modulated speech signal by means of the modulator 5. On the receiver side the frequency band located around the frequency of 2 kHz. is filtered away from the received frequency band by means of the filter 9. On the other hand, the synchronizing signal of 2 kHz. is obtained through a band pass filter 14 and is supplied to the PCM-coder 10 of the receiver in order to synchronize it with the PCM-decoder 6 of the transmitter by means of the zero crossings of the synchronizing signal.

FIG. 2 shows the synchronizing signal alone as indicated by a and superposed on the ciphered speech signal b2 as indicated by c. It appears that with the assumed values, four samplings are carried out during each period of the synchronizing frequency and that zero crossings can be used for synchronization. From FIG. 2 it is evident that the amplitude of the synchronizing signal also can be used for automatic volume control. In the modulator 5 the amplitude value of the synchronizing frequency is algebraically added to each sampling value. This implies that second sampling has an unchanged value while the intermediate values are added to or subtracted from the peak value of the synchronizing frequency. The output signal from the filter 14 is also supplied to an automatic volume control device 15 controlled by the maximum amplitude values of the synchronizing signal, so that an adjustment of the volume can be carried out.

The synchronizing signal can be added to the speech signal also after the PCM-decoder as is easy to see.

In order to avoid synchronization errors in transmission systems with simple side-band and suppressed carrier wave, due to a certain slight difference between the original carrier wave and the carrier wave generated in the receiver it may be suitable to use, instead of a synchronizing signal, the difference between two signals. In this case two signals are transmitted, for example, the frequencies 2500 'Hz. and 500 Hz. with a sampling frequency value chosen to be 8 kHz. according to the embodiment, so that the difference between the two frequencies always will be 2 kHz. on the receiver side irrespectively of a possible deviation between the carrier frequency oscillator of the transmitter and of the receiver.

It is evident that the expression pulse code can either be PCM-modulation as described in connection with the embodiment as well as delta modulation.

Hereinafter, the term analog signals will be used to mean speech signals, audio signals or the like.

I claim:

1. The method of transmitting from a transmitter to a receiver analog signals in ciphered form comprising the steps, at the transmitter, of converting first analog signals into first coded combinations of pulse signals, enciphering said first coded combinations of pulse signals into second coded combinations of pulse signals, converting said second coded combinations of signals to second analog signals, transmitting said second analog signals from the transmitter to the receiver, and at the receiver converting said second analog signals to said second coded combinations of pulse signals, deciphering said second coded combinations of pulse signals to said first coded combinations of pulse signals, and converting said first coded combinations of pulse signals to said first analog signals.

2. The method of claim 1 wherein the converting of the first analog signals at the transmitter occurs at a given sampling frequency, and the converting of the second analog signals at the receiver occurs at the given sampling frequency, further comprisng the steps of introducing at least one sinusoidal synchronizing signal having a frequency which is a submultiple of said given frequency to the signals transmitted from the transmitter to the receiver so that the zero crossings of the synchronizing signals control the synchronization of the conversion at the receiver.

3. The method of claim 2 wherein said sinusoidal synchronizing signal is added to the signals at the receiver in such a way that at least the minimum, maximum and zero amplitudes of the synchronizing signals are added to said signals at the receiver.

4. The method of claim 1 wherein the converting of the first analog signals at the transmitter occurs at a given sampling frequency, and the converting of the second analog signals at the receiver occurs at a given sampling frequency, further comprising the step of introducing at least two synchronizing signals to the signals transmitted from the transmitter to the receiver, said two synchronizing signals having frequencies such that their difference frequency is a submultiple of said given frequency so that a difference frequency signal derived from said two synchronizing signals synchronizes the converting of the second analog signals at the receiver.

5. Method according to claim 2, characterized thereby that said synchronizing signal is added to the signals at the transmitter after conversion to the second analog signals.

6. Method according to claim 2, characterized thereby that said synchronizing signal is added to the first coded combinations of pulse signals.

7. A system for transmitting ciphered information comprising: a transmitter including means for encoding audio signals to first pulse coded signals, means for enciphering said first pulse coded signals to second pulse coded signals, means for converting said second pulse coded signals to scrambled audio signals, and means for transmitting said scrambled audio signals; and a receiver for receiving said scrambled audio signals, said receiver including means for encoding the received scrambled audio signals to substantially said second pulse coded signals, means for deciphering said second pulse coded signals to said first pulse coded signals, and means for converting said first pulse coded signals to audio signals.

8. The system of claim 7 further comprising means at the transmitter for introducing a synchronizing signal to said scrambled audio signals, and means at said receiver for utilizing said synchronizing signal to synchro nize the operation of said encoder means therein.

9. The system of claim 8 wherein said encoder means sample the audio signals at a given frequency and said synchronizing signal has a frequency which is a submultiple of said given frequency, and said utilizing means includes filter means for extracting said synchronizing signal from the received scrambled audio signals to synchronize the sampling frequency of said encoder means therein.

10. The system according to claim 8, characterized thereby that the transmitter further comprises a filter device which filters out a narrow frequency band located around the frequency of the synchronizing signal, from the original audio signal.

11. The system according to claim 8, characterized thereby that the receiver further comprises an automatic volume control device supplied by the synchronizing signal for forming a control signal proportional to the peak value of the synchronizing signal for regulating the amplitude of the incoming scrambled audio signal.

References Cited UNITED STATES PATENTS 2,777,897 1/1957 Gretener et al. 325-32 RICHARD A. FARLEY, Primary Examiner C. E. WANDS, Assistant Examiner US. Cl. X.R. 1791l5; 325--34 

